(a) Field of the Invention
The present invention relates to a method for controlling an automatic transmission and, more particularly, to a method for controlling manual downshifting in an automatic transmission which allows a driver to downshift the automatic transmission by manipulating a shift lever without experiencing shift shock.
(b) Description of the Related Art
Automatic transmissions are typically equipped with a transmission control unit (TCU) for automatically shifting the transmission in response to sensed operating parameters such as vehicle speed. The TCU controls a plurality of clutches and brakes by selectively applying or releasing hydraulic pressure to effect changes in a drive mode in accordance with driver manipulation of a shift lever, in addition to realizing different speeds in a drive D range.
FIG. 1 shows a hydraulic circuit diagram of a hydraulic control system in the conventional automatic transmission. In FIG. 1, reference numeral C1 denotes a second brake, C2 an under-drive clutch, C3 an over-drive clutch, C4 a reduction brake, C5 a direct clutch, C6 a reverse clutch, and C7 a low reverse brake.
Reference numeral 100 in FIG. 1 denotes a manual valve (M/V), 21 a low reverse solenoid valve (L/R-S/V), 31 a second solenoid valve (2ND-SNV), 41 an under-drive solenoid valve (U/D-S/V), 51 an over-drive solenoid valve (O/D-S/V), and 61 a reduction solenoid valve (RED-S/V).
Reference numeral 22 denotes a low reverse pressure control valve (L/R-PCV), 32 a second pressure control valve (2.sup.ND -PCV), 42 an underclutch pressure control valve (U/D-PCV), 52 an overdrive pressure control valve (O/D-PCV), 62 a reduction brake pressure control valve (RED-PCV), 71 a fail-safe valve-A (FSV-A), 72 a fail-safe valve-B (FSV-B), 73 a fail-safe valve-C (FSV-C), and 81 an exhaust check valve (EX-V).
FIG. 2 is a block diagram showing a typical power-on manual downshift control system of the automatic transmission. As shown in FIG. 2, the power-on manual downshift control system for automatic transmissions includes a driving state detector 10 for detecting parameters indicative of the driving state of the vehicle from various sensors, the driving state detector 10 producing and outputting electrical signals corresponding to the driving state of the vehicle; a TCU 20 which receives the electrical signals from the driving state detector 10 and responsively produces duty control signals for manual downshifting on the basis of the electrical signals; and an actuator 30 controlling the transmission gear ratio in accordance with the duty control signals from the TCU 20 by applying and releasing line pressures to and from various lines of the hydraulic control system.
The driving state detector 10 includes an output rpm sensor 11 for sensing rpm of an output shaft and responsively producing a corresponding electrical signal; a throttle opening sensor 12 for sensing a throttle opening degree and responsively producing a corresponding electrical signal; a shift lever sensor 13 for sensing a position of a shift lever, which is manipulated by the driver, and responsively producing a corresponding electrical signal; an engine rpm sensor 14 for sensing engine rpm and responsively producing a corresponding electrical signal; an accelerator pedal sensor 15 for sensing an amount of displacement of an accelerator pedal and responsively producing a corresponding electrical signal; and an air intake sensor 16 for sensing an amount of air entering the engine and responsively producing a corresponding electrical signal.
The actuator 30 includes the L/R-S/V 21, 2.sup.ND -S/V 31, U/D-S/V 41, O/D-S/V 51, and RED-S/V 61.
A conventional power-on manual downshift control method in an automatic transmission provided with the above power-on manual downshift control system will be described hereinafter. For ease of description, only one manual downshift operation, i.e. a 5.fwdarw.4 power-on manual downshift operation, will be described with reference to FIGS. 6a, 6b, 6c, 6d, and 7.
The power-on manual downshift operation is performed when a sports mode, manual downshift mode, or power state mode, in each of which engine output is maximized, is activated at a shift begin (SB) point. The SB point is calculated on the basis of engine torque which is a function of vehicle speed.
With reference to FIG. 1, the second brake C1, overdrive clutch C3, and reverse clutch C6 are the operating friction elements for a fifth speed in the drive D range, while the underdrive clutch C2, overdrive clutch C3, and reverse clutch C6 act as operating elements in a fourth speed in the drive D range of the automatic transmission.
In the automatic transmission provided with the power-on manual downshift control system as described above, if the driver manipulates the shift lever in order to manually select the fourth speed of the drive D range, the TCU 20 receives a downshift signal from the shift lever sensor 13 of the driving state detector 10, and responsively produces a duty control signal. The actuator 30 receives the transmission control signal from the TCU 20 and responsively controls the transmission gear ratio to disengage the fifth speed and engage the fourth speed.
If the downshift signal is received, the TCU 20 transmits a duty control signal having a duty ratio of 0% to the 2ND-SV 31, which acts as the actuator 30, to release hydraulic pressure acting on the second brake C1. The transmission of this duty control signal by the TCU 20 is continued until the shift begin (SB) point is detected by engine rpm sensor 14.
The SB point is detected when a present turbine rpm (Nt(n)) is greater than a previous turbine rpm (Nt(n-1)). That is, if the duty ratio is changed from 100% to 0%, the engine torque increases such that the SB point is detected.
If the SB point is detected, the TCU 20 sends a duty control signal of a predetermined duty ratio (Dr) to the 2ND-SV 31, which acts as the actuator 30, so that feedback control for gradually releasing the line pressure applied to the second brake C1 is performed. This operation is continued until all the line pressure acting on the second brake C1 has been released, thereby completing manual downshift control.
While the feedback control is being performed, if it is detected that line pressure is being applied to the second brake C1 the TCU 20 sends another duty control signal to the U/D-S/V 41 such that line pressure is applied to the underdrive clutch C2.
The duty ratio (Dr) for releasing the line pressure applied to the second brake C1 is calculated using equation 1 as follows: EQU Dr=(Dr.sub.0 +Dr.sub.1 -Drkd).times.Ke+.DELTA.Dt, &lt;Equation 1&gt;
where
Dr is a feedback start duty ratio, PA1 Dr.sub.0 is an initial standard duty ratio (fixed value stored in memory), PA1 Dr.sub.1 is a learned compensating rate, PA1 Ke is an engine rpm calibration factor, PA1 .DELTA.Dt is an oil temperature compensating duty ratio, and PA1 Drkd is an amount by which a ratio between intake air to engine rpm is less than a preset standard value.
However, in the conventional power-on manual shift control process, the manual shift is usually performed when the throttle valve opening (Th) is over 50% such that the initial standard duty ratio (Dr.sub.0) for calculating the feedback start duty ratio (Dr) is set at a high input torque. Accordingly, in the case where the manual shift control is performed when the throttle opening (Th) is 20%-30%, the feedback control is performed at the same duty ratio (Dr) where the throttle valve opening is over 50%, even though the input torque is low.
In this case, since the manual shift control is performed using a high level of hydraulic pressure as when the input torque is high, shift shock occurs, as shown in FIG. 7, at the end of the manual shift operation such that shift quality is deteriorated.